Outline Servlets Overview of servlet technology First servlets Handling the client request Form data HTTP request headers Generating the server response HTTP status codes HTTP response headers Handling cookies Session tracking CS 4390 Web Programming Servlets 2 Overview Functions of Servlets The Advantages of Servlets Over Traditional CGI Compiling and Invoking Servlets Servlet Examples The Servlet Life Cycle Functions of Servlets Read explicit data sent by client (form data) Read implicit data sent by client (request headers) Generate the results Send the explicit data back to client (HTML) Send the implicit data to client (status codes and response headers) CS 4390 Web Programming Servlets 3 CS 4390 Web Programming Servlets 4 The Advantages of Servlets Over Traditional CGI Efficient Threads instead of OS processes, one servlet copy, persistence Convenient Lots of high-level utilities Powerful Sharing data, pooling, persistence Portable Run on virtually all operating systems and servers Secure No shell escapes, no buffer overflows Inexpensive CS 4390 Web Programming Servlets 5 Why Build Pages Dynamically? Web page is based on data submitted by the user E.g., results page from search engines and orderconfirmation pages at on-line stores Web page is derived from data that changes frequently E.g., a weather report or news headlines page Web page uses information from databases or other server-side sources E.g., an e-commerce site could use a servlet to build a Web page that lists the current price and availability of each item that is for sale CS 4390 Web Programming Servlets 6 1
Extending the Power of Servlets: JSP Idea: Use regular HTML for most of page Mark dynamic content with special tags <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML> <HEAD><TITLE>Welcome to Our Store</TITLE></HEAD> <BODY> <H1>Welcome to Our Store</H1> <SMALL>Welcome, <!-- User name is "New User" for first-time visitors --> <%= Utils.getUserNameFromCookie(request) %> To access your account settings, click <A HREF="Account-Settings.html">here.</A></SMALL><P> Regular HTML for rest of on-line store s Web page </BODY></HTML> CS 4390 Web Programming Servlets 7 Free Servlet and JSP Engines Apache Tomcat http://jakarta.apache.org/tomcat/ See http://archive.coreservlets.com/using-tomcat.html Allaire/Macromedia JRun http://www.allaire.com/products/jrun/ New Atlanta ServletExec http://www.servletexec.com/ Gefion Software LiteWebServer http://www.gefionsoftware.com/litewebserver/ Caucho's Resin http://www.caucho.com/ CS 4390 Web Programming Servlets 8 Compiling and Invoking Servlets Set your CLASSPATH Servlet JAR file (e.g., install_dir/lib/servlet.jar). Top of your package hierarchy Compiling and Invoking Servlets Put your servlet classes in proper location Locations vary from server to server. E.g., tomcat_install_dir/webapps/root/web-inf/classes See http://archive.coreservlets.com/using-tomcat.html jrun_install_dir/servers/default/default-app/ WEB-INF/classes Invoke your servlets http://host/servlet/servletname Custom URL-to-servlet mapping (via web.xml) CS 4390 Web Programming Servlets 9 CS 4390 Web Programming Servlets 10 A Simple Servlet That Generates Plain Text import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet { out.println("hello World"); Generating HTML Set the Content-Type header Use response.setcontenttype Output HTML Be sure to include the DOCTYPE Use an HTML validation service http://validator.w3.org/ http://www.htmlhelp.com/tools/validator/ CS 4390 Web Programming Servlets 11 CS 4390 Web Programming Servlets 12 2
public class HelloWWW extends HttpServlet { String doctype = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">\n"; out.println(doctype + "<HTML>\n" + "<HEAD><TITLE>Hello WWW</TITLE></HEAD>\n"+ "<BODY>\n" + "<H1>Hello WWW</H1>\n" + "</BODY></HTML>"); Some Simple HTML-Building Utilities public class ServletUtilities { public static final String DOCTYPE = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">"; public static String headwithtitle(string title) { return(doctype + "\n" + "<HTML>\n" + "<HEAD><TITLE>" + title + "</TITLE></HEAD>\n");... CS 4390 Web Programming Servlets 13 CS 4390 Web Programming Servlets 14 Some Simple HTML-Building Utilities Don t go overboard Complete HTML generation packages usually not worth the bother The JSP framework is a better solution CS 4390 Web Programming Servlets 15 Packaging Servlets Move the files to a subdirectory that matches the intended package name E.g. use the cwp package. So, the class files need to go in a subdirectory called cwp. Insert a package statement in the class file E.g., top of SimplerHelloWWW.java: package cwp; Set CLASSPATH to include top-level development directory Same as with any Java programming. Include package name in URL http://localhost/servlet/cwp.simplerhellowww CS 4390 Web Programming Servlets 16 HelloWWW with ServletUtilities and Packages SimplerHelloWWW Result package cwp; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SimplerHelloWWW extends HttpServlet { out.println(servletutilities.headwithtitle("hello WWW")+ "<BODY>\n" + "<H1>Hello WWW</H1>\n" + "</BODY></HTML>"); CS 4390 Web Programming Servlets 17 CS 4390 Web Programming Servlets 18 3
The Servlet Life Cycle init Executed once when the servlet is first loaded. Not called for each request service Called in a new thread by server for each request. Dispatches to doget, dopost, etc. Don t override this method! doget, dopost, doxxx Handles GET, POST, etc. requests Override these methods to provide desired behavior destroy Called when server deletes servlet instance. Not called after each request CS 4390 Web Programming Servlets 19 Why You Should Not Override service You can add support for other types of requests by adding doput, dotrace, etc. You can add support for modification dates Add a getlastmodified method The service method gives you automatic support for: HEAD, OPTIONS, and TRACE requests Alternative: have dopost call doget public void dopost(httpservletrequest request, { doget(request, response); CS 4390 Web Programming Servlets 20 Initializing Servlets Common in real-life servlets E.g., initializing database connection pools. Use ServletConfig.getInitParameter to read initialization parameters Call getservletconfig to obtain the ServletConfig object Set init parameters in web.xml (ver 2.2/2.3) /WEB-INF/web.xml Many servers have custom interfaces to create web.xml It is common to use init even when you don t read init parameters E.g., to set up data structures that don t change during the life of the servlet, to load information from disk, etc. CS 4390 Web Programming Servlets 21 A Servlet That Uses Initialization Parameters public class ShowMessage extends HttpServlet { private String message; private String defaultmessage = "No message."; private int repeats = 1; public void init() throws ServletException { ServletConfig config = getservletconfig(); message = config.getinitparameter("message"); if (message == null) { message = defaultmessage; try { String repeatstring = config.getinitparameter("repeats"); repeats = Integer.parseInt(repeatString); catch(numberformatexception nfe) { CS 4390 Web Programming Servlets 22 ShowMessage Servlet (Continued) String title = "The ShowMessage Servlet"; out.println(servletutilities.headwithtitle(title)+ "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=CENTER>" + title + "</H1>"); for(int i=0; i<repeats; i++) { out.println(message + "<BR>"); out.println("</body></html>"); CS 4390 Web Programming Servlets 23 Setting Init Parameters (Ver. 2.2 and Later)...\WEB-INF\web.xml tomcat_install_dir\webapps\root\web-inf\web.xml jrun_install_dir\servers\default\default-app\web-inf\web.xml <web-app> <servlet> <servlet-name>showmsg</servlet-name> <servlet-class>cwp.showmessage</servlet-class> <init-param><param-name>message</param-name> <param-value>shibboleth</param-value> </init-param> <init-param><param-name>repeats</param-name> <param-value>5</param-value> </init-param> </servlet> </web-app> CS 4390 Web Programming Servlets 24 4
ShowMessage Result Debugging Servlets Note use of registered name http://host/servlet/registeredname, not http://host/servlet/packagename.realname CS 4390 Web Programming Servlets 25 Use print statements; run server on desktop Integrated debugger in IDE Look at the HTML source Return error pages to the client Plan ahead for missing/malformed data Use the log file log("message") or log("message", Throwable) Look at the request data separately See EchoServer at archive.corewebprogramming.com Look at the response data separately See WebClient at archive.corewebprogramming.com Stop and restart the server CS 4390 Web Programming Servlets 26 Handling the Client Request: Form Data Handling the Client Request: Form Data Example URL at online travel agent http://host/path?user=marty+hall&origin=iad&dest=nrt Names (user) come from HTML author; values (Marty+Hall) usually come from end user Parsing form (query) data in traditional CGI Read the data one way for GET requests, another way for POST requests Chop pairs at &, then separate parameter names (left of the "=") from parameter values (right of the "=") URL decode values (e.g., "%7E" becomes "~") Need special cases for omitted values (param1=val1¶m2=¶m3=val3) and repeated params (param1=val1¶m2=val2¶m1=val3) CS 4390 Web Programming Servlets 28 Reading Form Data (Query Data) getparameter("name") Returns value as user entered it. I.e., URL-decoded value of first occurrence of name in query string. Works identically for GET and POST requests Returns null if no such parameter is in query getparametervalues("name") Returns an array of the URL-decoded values of all occurrences of name in query string Returns a one-element array if param not repeated Returns null if no such parameter is in query getparameternames() Returns Enumeration of request params CS 4390 Web Programming Servlets 29 An HTML Form With Three Parameters <FORM ACTION="/servlet/cwp.ThreeParams"> First Parameter: <INPUT TYPE="TEXT" NAME="param1"><BR> Second Parameter: <INPUT TYPE="TEXT" NAME="param2"><BR> Third Parameter: <INPUT TYPE="TEXT" NAME="param3"><BR> <CENTER><INPUT TYPE="SUBMIT"></CENTER> </FORM> CS 4390 Web Programming Servlets 30 5
Reading the Three Parameters Reading Three Parameters: Result public class ThreeParams extends HttpServlet { String title = "Reading Three Request Parameters"; out.println(servletutilities.headwithtitle(title) + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=CENTER>" + title + "</H1>\n" + "<UL>\n" + " <LI><B>param1</B>: " + request.getparameter("param1") + "\n" + " <LI><B>param2</B>: " + request.getparameter("param2") + "\n" + " <LI><B>param3</B>: " + request.getparameter("param3") + "\n" + "</UL>\n" + "</BODY></HTML>"); CS 4390 Web Programming Servlets 31 CS 4390 Web Programming Servlets 32 Filtering Strings for HTML-Specific Characters You cannot safely insert arbitrary strings into servlet output < and > can cause problems anywhere & and " cause problems inside of HTML attributes You sometimes cannot manually translate String is derived from a program excerpt or another source where it is already in standard format String is derived from HTML form data Failing to filter special characters makes you vulnerable to cross-site scripting attack http://www.cert.org/advisories/ca-2000-02.html http://www.microsoft.com/technet/security/crssite.asp See filter method of ServletUtilities at http://www.corewebprogramming.com Filtering Strings Method public static String filter(string input) { StringBuffer filtered = new StringBuffer(input.length()); char c; for(int i=0; i<input.length(); i++) { c = input.charat(i); if (c == '<') { filtered.append("<"); else if (c == '>') { filtered.append(">"); else if (c == '"') { filtered.append("""); else if (c == '&') { filtered.append("&"); else { filtered.append(c); return(filtered.tostring()); CS 4390 Web Programming Servlets 33 CS 4390 Web Programming Servlets 34 Handling the Client Request: HTTP Request Headers Handling the Client Request: HTTP Request Headers Example HTTP 1.1 Request GET /search?keywords=servlets+jsp HTTP/1.1 Accept: image/gif, image/jpg, */* Accept-Encoding: gzip Connection: Keep-Alive Cookie: userid=id456578 Host: www.somebookstore.com Referer: http://www.somebookstore.com/findbooks.html User-Agent: Mozilla/4.7 [en] (Win98; U) Understand HTTP to be effective with servlets or JSP CS 4390 Web Programming Servlets 36 6
Reading Request Headers Printing All Headers General getheader getheaders getheadernames Specialized getcookies getauthtype and getremoteuser getcontentlength getcontenttype getdateheader getintheader Related info getmethod, getrequesturi, getprotocol CS 4390 Web Programming Servlets 37 public class ShowRequestHeaders extends HttpServlet { String title = "Servlet Example: Showing Request Headers"; out.println(servletutilities.headwithtitle(title) + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=CENTER>" + title + "</H1>\n" + "<B>Request Method: </B>" + request.getmethod() + "<BR>\n" + "<B>Request URI: </B>" + request.getrequesturi() + "<BR>\n" + "<B>Request Protocol: </B>" + request.getprotocol() + "<BR><BR>\n" + CS 4390 Web Programming Servlets 38 Printing All Headers Printing All Headers: Netscape Result "<TABLE BORDER=1 ALIGN=CENTER>\n" + "<TR BGCOLOR=\"#FFAD00\">\n" + "<TH>Header Name<TH>Header Value"); Enumeration headernames = request.getheadernames(); while(headernames.hasmoreelements()) { String headername = (String)headerNames.nextElement(); out.println("<tr><td>" + headername); out.println(" <TD>" + request.getheader(headername)); out.println("</table>\n</body></html>"); public void dopost(httpservletrequest request, doget(request, response); CS 4390 Web Programming Servlets 39 CS 4390 Web Programming Servlets 40 Printing All Headers: Internet Explorer Result CS 4390 Web Programming Servlets 41 Common HTTP 1.1 Request Headers Accept Indicates MIME types browser can handle Can send different content to different clients Accept-Encoding Indicates encodings (e.g., gzip) browser can handle See following example Authorization User identification for password-protected pages. Instead of HTTP authorization, use HTML forms to send username/password. Store in session object. For details on programming security manually and using web.xml to tell the server to enforce security automatically. CS 4390 Web Programming Servlets 42 7
Common HTTP 1.1 Request Headers Connection In HTTP 1.0, keep-alive means browser can handle persistent connection. In HTTP 1.1, persistent connection is default. Persistent connections mean that the server can reuse the same socket over again for requests very close together from the same client Servlets can't do this unilaterally; the best they can do is to give the server enough info to permit persistent connections. So, they should set Content-Length with setcontentlength (using ByteArrayOutputStream to determine length of output). See example in Core Servlets and JavaServer Pages. Cookie Gives cookies previously sent to client. Use getcookies, not getheader. CS 4390 Web Programming Servlets 43 Common HTTP 1.1 Request Headers (Continued) Host Indicates host given in original URL This is a required header in HTTP 1.1. This fact is important to know if you write a custom HTTP client (e.g., WebClient used in book) or telnet to a server and use the HTTP/1.1 version If-Modified-Since Indicates client wants page only if it has been changed after specified date Don t handle this situation directly; implement getlastmodified instead. See example in Core Servlets and JavaServer Pages Chapter 2 CS 4390 Web Programming Servlets 44 Common HTTP 1.1 Request Headers (Continued) Referer URL of referring Web page Useful for tracking traffic; logged by many servers Can be easily spoofed User-Agent String identifying the browser making the request Use sparingly Again, can be easily spoofed CS 4390 Web Programming Servlets 45 Sending Compressed Pages: EncodedPage.java String encodings = request.getheader("accept-encoding"); String encodeflag = request.getparameter("encoding"); PrintWriter out; String title; if ((encodings!= null) && (encodings.indexof("gzip")!= -1) &&!"none".equals(encodeflag)) { title = "Page Encoded with GZip"; OutputStream out1 = response.getoutputstream(); out = new PrintWriter(new GZIPOutputStream(out1), false); response.setheader("content-encoding", "gzip"); else { title = "Unencoded Page"; out = response.getwriter(); CS 4390 Web Programming Servlets 46 EncodedPage.java out.println(servletutilities.headwithtitle(title) + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=CENTER>" + title + "</H1>\n"); String line = "Blah, blah, blah, blah, blah. " + "Yadda, yadda, yadda, yadda."; for(int i=0; i<10000; i++) { out.println(line); out.println("</body></html>"); out.close(); Sending Compressed Pages: Results Uncompressed (28.8K modem), Netscape 4.7 and Internet Explorer 5.0: > 50 seconds Compressed (28.8K modem), Netscape 4.7 and Internet Explorer 5.0: < 5 seconds Caution: be careful about generalizing benchmarks CS 4390 Web Programming Servlets 47 CS 4390 Web Programming Servlets 48 8
Generating the HTTP Response Generating the Server Response: HTTP Status Codes Example HTTP 1.1 Response HTTP/1.1 200 OK Content-Type: text/html <!DOCTYPE...> <HTML>... </HTML> Changing the status code lets you perform a number of tasks not otherwise possible Forward client to another page Indicate a missing resource Instruct browser to use cached copy Set status before sending document CS 4390 Web Programming Servlets 50 Setting Status Codes public void setstatus(int statuscode) Use a constant for the code, not an explicit int. Constants are in HttpServletResponse Names derived from standard message. E.g., SC_OK, SC_NOT_FOUND, etc. public void senderror(int code, String message) Wraps message inside small HTML document public void sendredirect(string url) Relative URLs permitted in 2.2/2.3 Also sets Location header CS 4390 Web Programming Servlets 51 Common HTTP 1.1 Status Codes 200 (OK) Everything is fine; document follows Default for servlets 204 (No Content) Browser should keep displaying previous document 301 (Moved Permanently) Requested document permanently moved elsewhere (indicated in Location header) Browsers go to new location automatically CS 4390 Web Programming Servlets 52 Common HTTP 1.1 Status Codes (Continued) 302 (Found) Requested document temporarily moved elsewhere (indicated in Location header) Browsers go to new location automatically Servlets should use sendredirect, not setstatus, when setting this header. See example 401 (Unauthorized) Browser tried to access protected page without proper Authorization header. See example in book 404 (Not Found) No such page. Servlets should use senderror to set this header Problem: Internet Explorer 5.0 CS 4390 Web Programming Servlets 53 A Front End to Various Search Engines: Code String searchstring = request.getparameter("searchstring"); if ((searchstring == null) (searchstring.length() == 0)) { reportproblem(response, "Missing search string."); return; searchstring = URLEncoder.encode(searchString); String numresults = request.getparameter("numresults");... String searchengine = request.getparameter("searchengine"); CS 4390 Web Programming Servlets 54 9
A Front End to Various Search Engines: Code (Continued) Front End to Search Engines: Result of Legal Request SearchSpec[] commonspecs = SearchSpec.getCommonSpecs(); for(int i=0; i<commonspecs.length; i++) { SearchSpec searchspec = commonspecs[i]; if (searchspec.getname().equals(searchengine)) { String url = searchspec.makeurl(searchstring, numresults); response.sendredirect(url); return; reportproblem(response, "Unrecognized search engine."); private void reportproblem(httpservletresponse response, String message) throws IOException { response.senderror(response.sc_not_found, "<H2>" + message + "</H2>"); CS 4390 Web Programming Servlets 55 CS 4390 Web Programming Servlets 56 Front End to Search Engines: Result of Legal Request Front End to Search Engines: Result of Illegal Request CS 4390 Web Programming Servlets 57 Fix: Tools, Internet Options, Advanced Deselect "Show 'friendly' HTTP error messages" Not a real fix -- doesn't help unsuspecting users of your pages CS 4390 Web Programming Servlets 58 Generating the Server Response: HTTP Response Headers Purposes Give forwarding location Specify cookies Supply the page modification date Instruct the browser to reload the page after a designated interval Give the document size so that persistent HTTP connections can be used Designate the type of document being generated Etc. CS 4390 Web Programming Servlets 59 Setting Arbitrary Response Headers public void setheader(string headername, String headervalue) Sets an arbitrary header public void setdateheader(string name, long millisecs) Converts millis since 1970 to date in GMT format public void setintheader(string name, int headervalue) Prevents need to convert int to String addheader, adddateheader, addintheader Adds header instead of replacing CS 4390 Web Programming Servlets 60 10
Setting Common Response Headers setcontenttype Sets the Content-Type header. Servlets almost always use this header. See Table 19.1 (Common MIME Types). setcontentlength Sets the Content-Length header. Used for persistent HTTP connections. See Connection request header. addcookie Adds a value to the Set-Cookie header. See separate section on cookies. sendredirect Sets Location header (plus changes status code) CS 4390 Web Programming Servlets 61 Common HTTP 1.1 Response Headers Cache-Control (1.1) and Pragma (1.0) A no-cache value prevents browsers from caching page. Send both headers or check HTTP version Content-Encoding The way document is encoded. Browser reverses this encoding before handling document. See compression example earlier. Content-Length The number of bytes in the response See setcontentlength on previous slide Use ByteArrayOutputStream to buffer document so you can determine size. See detailed example in Core Servlets and JavaServer Pages CS 4390 Web Programming Servlets 62 Common HTTP 1.1 Response Headers Content-Type The MIME type of the document being returned. Use setcontenttype to set this header Expires The time at which document should be considered out-of-date and thus should no longer be cached Use setdateheader to set this header Last-Modified The time document was last changed. Don t set this header explicitly; provide a getlastmodified method instead. See example in Core Servlets and JavaServer Pages Chapter 2 CS 4390 Web Programming Servlets 63 Common HTTP 1.1 Response Headers Location The URL to which browser should reconnect. Use sendredirect instead of setting this directly. Refresh The number of seconds until browser should reload page. Can also include URL to connect to. See following example. Set-Cookie The cookies that browser should remember. Don t set this header directly; use addcookie instead. WWW-Authenticate The authorization type and realm needed in Authorization header. See details in More Servlets & JavaServer Pages. CS 4390 Web Programming Servlets 64 Persistent Servlet State and Auto-Reloading Pages Idea: generate list of large (e.g., 150-digit) prime numbers Show partial results until completed Let new clients make use of results from others Demonstrates use of the Refresh header Shows how easy it is for servlets to maintain state between requests Very difficult in traditional CGI Also illustrates that servlets can handle multiple simultaneous connections Each request is in a separate thread Synchronization required for shared data Generating Prime Numbers: Source Code int numprimes = ServletUtilities.getIntParameter(request, "numprimes", 50); int numdigits = ServletUtilities.getIntParameter(request, "numdigits", 120); // findprimelist is synchronized PrimeList primelist = findprimelist(primelistvector, numprimes, numdigits); if (primelist == null) { primelist = new PrimeList(numPrimes, numdigits, true); CS 4390 Web Programming Servlets 65 CS 4390 Web Programming Servlets 66 11
Generating Prime Numbers: Source Code synchronized(primelistvector) { if (primelistvector.size() >= maxprimelists) primelistvector.removeelementat(0); primelistvector.addelement(primelist); Vector currentprimes = primelist.getprimes(); int numcurrentprimes = currentprimes.size(); int numprimesremaining = (numprimes - numcurrentprimes); boolean islastresult = (numprimesremaining == 0); if (!islastresult) { response.setheader("refresh", "5"); // Show List of Primes found... CS 4390 Web Programming Servlets 67 Prime Number Servlet: Front End CS 4390 Web Programming Servlets 68 Prime Number Servlet: Initial Result Prime Number Servlet: Final Result CS 4390 Web Programming Servlets 69 CS 4390 Web Programming Servlets 70 The Potential of Cookies Handling Cookies Idea Servlet sends a simple name and value to client Client returns same name and value when it connects to same site (or same domain, depending on cookie settings) Typical Uses of Cookies Identifying a user during an e-commerce session Servlets have a higher-level API for this task Avoiding username and password Customizing a site Focusing advertising CS 4390 Web Programming Servlets 72 12
Cookies and Focused Advertising Some Problems with Cookies The problem is privacy, not security Servers can remember your previous actions If you give out personal information, servers can link that information to your previous actions Servers can share cookie information through use of a cooperating third party like doubleclick.net Poorly designed sites store sensitive information like credit card numbers directly in cookie Morals for servlet authors If cookies are not critical to your task, avoid servlets that totally fail when cookies are disabled Don't put sensitive info in cookies CS 4390 Web Programming Servlets 73 CS 4390 Web Programming Servlets 74 Sending Cookies to Browser Standard approach: Cookie c = new Cookie("name", "value"); c.setmaxage(...); // Means cookie persists on disk // Set other attributes. response.addcookie(c); Simplified approach: Use LongLivedCookie class: public class LongLivedCookie extends Cookie { public static final int SECONDS_PER_YEAR = 60*60*24*365; public LongLivedCookie(String name, String value) { super(name, value); setmaxage(seconds_per_year); CS 4390 Web Programming Servlets 75 Reading Cookies from Browser Standard approach: Cookie[] cookies = request.getcookies(); if (cookies!= null) { for(int i=0; i<cookies.length; i++) { Cookie c = cookies[i]; if (c.getname().equals("somename")) { dosomethingwith(c); break; CS 4390 Web Programming Servlets 76 Reading Cookies from Browser Simplified approach: Extract cookie or cookie value from cookie array by using ServletUtilities.getCookieValue or ServletUtilities.getCookie ServletUtilities.getCookieValue public static String getcookievalue(cookie[] cookies, String cookiename, String defaultval) { if (cookies!= null) { for(int i=0; i<cookies.length; i++) { Cookie cookie = cookies[i]; if (cookiename.equals(cookie.getname())) return(cookie.getvalue()); return(defaultval); The getcookie method is similar Returns the Cookie object instead of the value CS 4390 Web Programming Servlets 77 CS 4390 Web Programming Servlets 78 13
Simple Cookie-Setting Servlet Result of Cookie-Setting Servlet public class SetCookies extends HttpServlet { for(int i=0; i<3; i++) { Cookie cookie = new Cookie("Session-Cookie-" + i, "Cookie-Value-S" + i); response.addcookie(cookie); cookie = new Cookie("Persistent-Cookie-" + i, "Cookie-Value-P" + i); cookie.setmaxage(3600); response.addcookie(cookie); out.println(...); CS 4390 Web Programming Servlets 79 CS 4390 Web Programming Servlets 80 Simple Cookie-Viewing Servlet Simple Cookie-Viewing Servlet public class ShowCookies extends HttpServlet { String title = "Active Cookies"; out.println(servletutilities.headwithtitle(title) + "<BODY BGCOLOR=\"#FDF5E6\">\n" + "<H1 ALIGN=\"CENTER\">" + title + "</H1>\n" + "<TABLE BORDER=1 ALIGN=\"CENTER\">\n" + "<TR BGCOLOR=\"#FFAD00\">\n" + " <TH>Cookie Name\n" + " <TH>Cookie Value"); CS 4390 Web Programming Servlets 81 Cookie[] cookies = request.getcookies(); if (cookies!= null) { Cookie cookie; for(int i=0; i<cookies.length; i++) { cookie = cookies[i]; out.println("<tr>\n" + " <TD>" + cookie.getname() + "\n" + " <TD>" + cookie.getvalue()); out.println("</table></body></html>"); CS 4390 Web Programming Servlets 82 Result of Cookie-Viewer (Before & After Restarting Browser) Methods in the Cookie API getdomain/setdomain Lets you specify domain to which cookie applies. Current host must be part of domain specified getmaxage/setmaxage Gets/sets the cookie expiration time (in seconds). If you fail to set this, cookie applies to current browsing session only. See LongLivedCookie helper class given earlier getname/setname Gets/sets the cookie name. For new cookies, you supply name to constructor, not to setname. For incoming cookie array, you use getname to find the cookie of interest CS 4390 Web Programming Servlets 83 CS 4390 Web Programming Servlets 84 14
Methods in the Cookie API (Continued) getpath/setpath Gets/sets the path to which cookie applies. If unspecified, cookie applies to URLs that are within or below directory containing current page getsecure/setsecure Gets/sets flag indicating whether cookie should apply only to SSL connections or to all connections getvalue/setvalue Gets/sets value associated with cookie. For new cookies, you supply value to constructor, not to setvalue. For incoming cookie array, you use getname to find the cookie of interest, then call getvalue on the result Session Tracking CS 4390 Web Programming Servlets 85 Session Tracking The Session Tracking API Why? When clients at an on-line store add an item to their shopping cart, how does the server know what s already in the cart? When clients decide to proceed to checkout, how can the server determine which previously created shopping cart is theirs? How? Cookies URL-rewriting Hidden form fields Higher-level API needed Session objects live on the server Automatically associated with client via cookies or URLrewriting Use request.getsession(true) to get either existing or new session Behind the scenes, the system looks at cookie or URL extra info and sees if it matches the key to some previously stored session object. If so, it returns that object. If not, it creates a new one, assigns a cookie or URL info as its key, and returns that new session object. Hashtable-like mechanism lets you store arbitrary objects inside session setattribute stores values getattribute retrieves values CS 4390 Web Programming Servlets 87 CS 4390 Web Programming Servlets 88 Using Sessions HttpSession Methods HttpSession session = request.getsession(true); ShoppingCart cart = (ShoppingCart)session.getAttribute("shoppin gcart"); if (cart == null) { // No cart already in session cart = new ShoppingCart(); session.setattribute("shoppingcart", cart); dosomethingwith(cart); CS 4390 Web Programming Servlets 89 getattribute, getvalue [2.1] Extracts a previously stored value from a session object. Returns null if no value is associated with given name setattribute, putvalue [2.1] Associates a value with a name. Monitor changes: values implement HttpSessionBindingListener. removeattribute, removevalue [2.1] Removes values associated with name getattributenames, getvaluenames [2.1] Returns names of all attributes in the session getid Returns the unique identifier CS 4390 Web Programming Servlets 90 15
HttpSession Methods (Continued) isnew Determines if session is new to client (not to page) getcreationtime Returns time at which session was first created getlastaccessedtime Returns time session was last sent from client getmaxinactiveinterval, setmaxinactiveinterval Gets or sets the amount of time session should go without access before being invalidated invalidate Invalidates the session and unbinds all objects associated with it CS 4390 Web Programming Servlets 91 A Servlet Showing Per-Client Access Counts String title = "Session Tracking Example"; HttpSession session = request.getsession(true); String heading; Integer accesscount = (Integer)session.getAttribute("accessCount"); if (accesscount == null) { accesscount = new Integer(0); heading = "Welcome, Newcomer"; else { heading = "Welcome Back"; accesscount = new Integer(accessCount.intValue() + 1); session.setattribute("accesscount", accesscount); CS 4390 Web Programming Servlets 92 First Visit to ShowSession Servlet Eleventh Visit to ShowSession Servlet CS 4390 Web Programming Servlets 93 CS 4390 Web Programming Servlets 94 Review: Getting Started Review: Servlets Servlets are efficient, portable, powerful, and widely accepted in industry Regardless of deployment server, run a free server on your desktop for development Getting started: Set your CLASSPATH Servlet and JSP JAR files Top of your package hierarchy Put class files in proper location.../web-inf/classes with servlets 2.2 Use proper URL; default is http://host/servlet/servletname Download existing servlet first time Start with HelloWWW from www.corewebprogramming.com CS 4390 Web Programming Servlets 96 16
Review: Getting Started (Continued) Main servlet code goes in doget or dopost: The HttpServletRequest contains the incoming information The HttpServletResponse lets you set outgoing information Call setcontenttype to specify MIME type Call getwriter to obtain a Writer pointing to client One-time setup code goes in init Servlet gets initialized and loaded once Servlet gets invoked multiple times Review: Handling Form Data (Query Data) Query data comes from HTML forms as URL-encoded name/value pairs Servlets read data by calling request.getparameter("name") Results in value as entered into form, not as sent over network. I.e. not URL-encoded. Always check for missing or malformed data Special case: query data that contains special HTML characters Need to be filtered if query data will be placed into resultant HTML page CS 4390 Web Programming Servlets 97 CS 4390 Web Programming Servlets 98 Review: Using HTTP Request Headers Many servlet tasks can only be accomplished by making use of HTTP headers coming from the browser Use request.getheader for arbitrary header Cookies, authorization info, content length, and content type have shortcut methods Most important headers you read directly Accept Accept-Encoding Connection Referer User-Agent CS 4390 Web Programming Servlets 99 Review: Generating the HTTP Response Many servlet tasks can only be accomplished through use of HTTP status codes and headers sent to the browser Two parts of the response Status line In general, set via response.setstatus In special cases, set via response.sendredirect and response.senderror Response headers In general, set via response.setheader In special cases, set via response.setcontenttype, response.setcontentlength, response.addcookie, and response.sendredirect CS 4390 Web Programming Servlets 100 Review: Generating the HTTP Response (Continued) Most important status codes 200 (default) 302 (forwarding; set via sendredirect) 401 (password needed) 404 (not found; set via senderror) Most important headers you set directly Cache-Control and Pragma Content-Encoding Content-Length Expires Refresh WWW-Authenticate CS 4390 Web Programming Servlets 101 Review: Handling Cookies Cookies involve name/value pairs sent from server to browser and returned when the same page, site, or domain is visited later Let you Track sessions (use higher-level API) Permit users to avoid logging in at low-security sites Customize sites for different users Focus content or advertising Setting cookies Cookie constructor, set age, response.addcookie Reading cookies Call request.getcookies, check for null, look through array for matching name, use associated value CS 4390 Web Programming Servlets 102 17
Review: Session Tracking Preview: The Need for JSP Although it usually uses cookies behind the scenes, the session tracking API is higher-level and easier to use than the cookie API Session information lives on server Cookie or extra URL info associates it with a user Obtaining session request.getsession(true) Associating values with keys session.setattribute Finding values associated with keys session.getattribute Always check if this value is null before trying to use it CS 4390 Web Programming Servlets 103 With servlets, it is easy to Read form data Read HTTP request headers Set HTTP status codes and response headers Use cookies and session tracking Share data among servlets Remember data between requests Get fun, high-paying jobs But, it sure is a pain to Use those println statements to generate HTML Maintain that HTML CS 4390 Web Programming Servlets 104 Preview: Benefits of JSP Although JSP technically can't do anything servlets can't do, JSP makes it easier to: Write HTML Read and maintain the HTML JSP makes it possible to: Use standard HTML tools such as HomeSite or UltraDev Have different members of your team do the HTML layout and the programming JSP encourages you to Separate the (Java TM technology) code that creates the content from the (HTML) code that presents it CS 4390 Web Programming Servlets 105 18